1. Field of the Invention
The present invention relates to the field of workholders or hold downs for machinery and specifically to such workholders which act to urge a workpiece, such as a board, against a fence or other guide.
2. Background Information
There are a wide variety of applications in which a substantially linear workpiece is fed into or through a machine which performs some operation on the workpiece. In carrying out this operation, it is common to use a fence, or other guide, to properly align the workpiece. A simple example is ripping a board on table saw. In order for the fence to be effective, the workpiece must be held tightly against the fence. While this may be done by hand, safety, accuracy, and productivity concerns often dictate the use of a workholder.
These workholders may be rollers, springs, featherboards, or any of a variety of similar devices. Their operation is relatively straight forward. A feather board for example is attached to the table of the machine, adjusted so that it contacts the edge of the board at the correct angle and with the desired pressure, and then locked down. Common to most of these workholders is that they must be adjusted to the width of the workpiece and reset if that width changes or another workpiece is substituted. While they can accommodate a small amount of variation in the width of the workpiece, such as an inch or so, they are not intended to accommodate large variation.
This is troublesome in several circumstances. The first is where the edge of the board being contacted by the workholder is irregular. This is common in rough cut lumber for example. The edge may be wavy, varying 2 or 3 inches (or more) along the length of the board. A conventional workholder which is set properly for the narrowest portion, will jamb at the widest portion. If set for the widest portion, it will lose contact with the narrowest portion, failing to perform its function. Even where the variation is within limits, wide variations in pressure on the workpiece may result, affecting feed rates and finish quality of the operation. A second circumstance is where the board tapers, perhaps by design. This is similar to the above, but may have a significantly greater range of width which must be accommodated. A third circumstance is where a series of strips is being removed from the fence side of the workpiece. At each pass, the workpiece becomes narrower, requiring the workholder be reset as often as each pass.
The last situation, above, can be addressed with conventional workholders, but can seriously impact the productivity, and enjoyment, of the operation. The first two situations can not be handled by conventional workholders, and performing the operation without the workholders may pose a serious safety risk by requiring hand guiding.
There is a need for a workholder for use with machinery which can accommodate relatively wide variations in the width of the workpiece, or variations in the contour of the edge which it contacts. It should also readily adapt to width variations between workpieces without requiring manual repositioning. It should maintain a relatively constant pressure against the workpiece despite these variations. Ideally its operation would require minimal intervention by the operator and be adaptable to circumstances or operator preferences.
The present invention is directed to a compliant workholder for use with a piece of machinery, the workholder preferably using one or more pneumatic pistons to press a feather board or other contact element against the workpiece with substantially constant pressure. The pistons(s) allow the contact element to move in and out, following the edge of the workpiece as its profile varies.
According to the invention there is provided a frame which is mounted to the piece of machinery, a guide rod which slides in the frame, a head mounted on the outer end of the rod, and a mechanism for pushing the head outward against the workpiece.
According to an aspect of the invention the head will mount a resilient contact element, such as a feather board to make contact with the workpiece.
According to another aspect of the invention the head is retracted when the user presses a switch and remains retracted for a user selectable period of time.
Further in accordance with the invention the head may be allowed to pivot, adjusting to the contours of the workpiece.
The advantages of such an apparatus are that the head maintains substantially constant pressure against the workpiece while extending and retracting to follow the contour of the workpiece. The resilient contact element, such as a feather board, adapts to more minor irregularities. The easy retraction simplifies insertion of the workpiece, eases user operation, and increases the likelihood that the workholder will be used. Workpiece to workpiece variations are also accommodated as the head will extend until the workpiece is contacted, or the movement limits reached.
The above and other features and advantages of the present invention will become more clear from the detailed description of a specific illustrative embodiment thereof, presented below in conjunction with the accompanying drawings.